1. Field of the Invention
The present invention relates to a hybrid construction machine having a motor generator to assist motive power of an engine and generate electric power.
2. Description of the Related Art
In recent years, a construction machine driven with a hydraulic system has a large engine selected in anticipation of operation under maximum load to perform all the operations under light to heavy load. However, an operation under heavy load is merely a part of the entire work with the construction machine. Accordingly, the engine performance is redundant upon work under light and intermediate load, which is not preferable in terms of fuel efficiency.
On the other hand, as disclosed in e.g. Japanese Patent No. 4800514, conventionally known is a hybrid construction machine having a hybrid system. In the construction machine, an engine is downsized, however, shortage of engine output accompanying downsizing of the engine is complemented with output torque of a motor generator having quick output response, to improve fuel efficiency.
However, in this hybrid construction machine, when engine assistance conditions such as engine speed and load are satisfied, engine motive power is assisted with the motor generator. Accordingly, even when assistance is not required (or is redundant), the assistance operation with the motor generator may be continued in some cases. In such case, electric power accumulated in a power storage device to supply/receive electric power to/from the motor generator is wasted, or over discharge from the power storage device is caused. It is not preferable in terms of energy saving. Further, there is a possibility that the life of the power storage device is shortened.
Accordingly, as disclosed in e.g. U.S. Pat. No. 7,728,449, a proposed conventional technique is a structure having a control unit to control motive-power assistance with a motor generator to an engine, and an operation state detection unit to detect an engine stall state or an operation state directly connected to engine stall. When the engine stall state or the operation state directly connected to engine stall has been detected with the operation state detection unit, the motive-power assistance with the motor generator to the engine is automatically stopped.
In this U.S. Pat. No. 7,728,449, the operation state detection unit includes an engine speed detection unit to detect an actual engine speed, and an assistance/stop determination unit to obtain a deviation between the actual engine speed detected with the engine speed detection unit and a target speed, and when the deviation is equal to or greater than a previously-set value, determine to stop the motive-power assistance to the engine.
In the hybrid construction machine having a motor generator as disclosed in the above-described conventional techniques, the power storage device has a large capacity. In the hybrid construction machine having the large capacity power storage device, when the hydraulic load required in the hydraulic system is lower than the maximum motive power of the motor generator upon operation under light or intermediate load, it is possible to support the hydraulic load only with the output torque of the motor generator even though the engine output torque is maintained in an extremely low state. In this situation, unnecessary engine motive-power assistance is performed with the motor generator. There is a possibility that motoring occurs, i.e., the engine does not effectively function and wastefully rotates with the output torque of the motor generator, and engine overspeed easily occurs. With this phenomenon, it is conceivable that a problem such as damage to the engine or thermal effect on the engine occurs.
Further, according to the conventional technique disclosed in the U.S. Pat. No. 7,228,449, when it is possible to support the hydraulic load only with the output torque of the motor generator as described above, it is possible to control the engine speed without causing a deviation between the actual engine speed detected with the engine speed detection unit and the target speed. Accordingly, since it is impossible for the operation state detection unit to appropriately detect engine stall or an operation state directly connected to the engine stall, it is not possible to stop unnecessary engine motive-power assistance with the assistance/stop determination unit. This might cause waste of electric power accumulated in the power storage device and over discharge from the power storage device.
Especially, in the hybrid construction machine, there are a comparatively small number of opportunities to recall regenerative energy in the power storage device. When the electric power accumulated in the power storage device is lowered, the engine is driven so as to charge the power storage device. The engine is loaded for the driving and fuel is used, thus the fuel efficiency in the entire work is degraded. Accordingly, it is necessary to ensure the electric power in the power storage device as much as possible even upon operation under light or intermediate load. Further, it is desired that the motor generator is efficiently used to realize energy saving.